Well Spica is definitely one of the better units you can get. Most other archers only get a measly "will target X type of unit as priority after range" passive which is largely ignoreable. Spica actually gets more damage to all archers and has great damage as well.

As for the question of when to Saki, that totally depends on your own team and your DC and what AW fodders you have available. If you haven't even started leveling the fodders yet, then what point is there to wait to awaken things instead of getting Saki? On the other hand if you have some core units or very very special units around that are maxed and you can awaken rather soon then Saki will be taking a lower priority.